Synopsis

Simultaneous
Multi-Contrast (SMC) Imaging enables a synchronous acquisition of multiple
image contrasts within one measurement. The technique reduces patient
examination times and facilitates accurate image registration between
contrasts. Previous work used readout-segmented EPI (rs-EPI) to perform high-resolution,
navigator-corrected, diffusion-weighted imaging simultaneously with a
T2*-weighted acquisition. This combination of contrasts has clinical
significance in acute stroke. These previous studies did not use in-plane
acceleration to reduce spatial distortion caused by the EPI readout. This study
introduces an updated version of the SMC technique that incorporates in-plane
acceleration with GRAPPA to allow an improved image quality for future clinical
studies.

Purpose

Simultaneous Multi-Contrast (SMC)
Imaging1,2 uses methodology from simultaneous-multi-slice (SMS)
imaging to enable the acquisition of multiple image contrasts within a single
measurement. Initial application has focused on the synchronous acquisition of
diffusion-weighted (DW) T2*-weighted (T2*W) images using a readout-segmented
(rs) echo planar (EPI) imaging sequence with navigator-based phase correction3.
This has particular clinical relevance for acute
stroke, where examination time is a critical factor4. A limitation
to previous studies was that in-plane parallel imaging was not used, resulting
in an EPI readout with a long effective echo spacing, leading to increased
spatial distortion. The purpose of the current study was to combine the SMC
technique with in-plane parallel imaging using GRAPPA5 to achieve an
improved image quality that is suitable for clinical studies.

Methods

Pulse Sequence:

The modified rs-EPI sequence for SMC
(Fig. 1), acquires DW and T2*W contrast from two slice positions at the same
time. A blipped-CAIPIRINHA6,7 gradient scheme along the slice-select
(GS) direction is used in conjunction with receiver phase modulation to shift
the T2*-weighted image by half a field of view (FOV) in the phase-encoding
direction. Finally, an RF refocusing pulse is applied to slice A only to
generate a 2D navigator signal to phase correct the DW imaging data. The
sequence was used firstly without in-plane acceleration and then with acceleration
factors (AF) of 2 and 4 to sample every second or every fourth phase-encoding
line respectively.

Data Acquisition:

Data were acquired from a healthy
subject at 3T (MAGNETOM Skyra, Siemens Healthcare GmbH, 20 channel-head coil)
with the following parameters: FOV 230mm, matrix 224x224, slice thickness 4mm,
TR 4500ms; (DW contrast) one scan at b-value of zero and three scans at
1000s/mm2 along phase-encode, readout and slice-select directions; (T2*W
contrast) flip angle 30°. Separate multi-slice reference data were acquired for
DW contrast with b=0 and for T2*W contrast for the SMC reconstruction. In
addition, low-resolution reference scans were acquired for the in-plane GRAPPA5
reconstruction.

Data Processing:

The slice-GRAPPA reconstruction7
with inter-slice leakage artefact reduction8 was used to separate
the aliased slices into single-slice data. Firstly, the SMC reference data were
used to fit separate 3 x 3 kernels. The same weight sets were used at all
b-values to separate DW and T2*W images. Secondly, the separated data were
passed to the standard in-plane GRAPPA reconstruction5. All data
processing was performed using the manufacturer’s proprietary image calculation
environment (ICE).

Results

The
top row of Fig. 2 shows the result of the simultaneous acquisition of DW data
with a b=0 and T2*W data from a separate slice position
without in-plane acceleration. The separated contrasts are displayed in the
second row. The lower section shows the result of an SMC acquisition with an
in-plane AF of two for a single diffusion-encoding direction and a b-value of
1000 s/mm2. Fig. 3 compares the result of calculated trace-weighted
images without in-plane acceleration to those with an AF of two and four. Fig.
4 shows the corresponding T2*W data with three averages.

Discussion

The separated
images in Figs. 2, 3, and 4 demonstrate the feasibility of combining SMC with
in-plane acceleration. Signals from the two contrast types are correctly
assigned to the correct slice positions without evidence of cross talk. A high
level of signal separation was achieved for both low and high b-value scans. The
benefit of using GRAPPA to reduce the EPI echo-train length is clearly
demonstrated by the reduced distortions in the images with in-plane
acceleration shown in Fig. 3. However, the images with an AF of four also show
an unacceptable loss in SNR. Fat suppression was performed by
slice-select-gradient polarity reversal for DW signals and by water excitation
for T2*W signals. In both cases these techniques proved to be
inadequate at high in-plane acceleration factors, which requires attention in
future work. Further work is also required to address the motion-induced signal
losses seen in some images because the current implementation of the sequence
does not use a reacquisition scheme to avoid uncorrectable, severe phase
errors, as described in previous studies using DW rs-EPI3.

Conclusion

This study has demonstrated
that SMC can be combined with in-plane parallel imaging to allow the
simultaneous acquisition of DW and T2*W images with reduced spatial
distortion. The resulting technique promises to be of particular interest for
reducing overall measurement times in acute stroke when TR values are too short
for SMS techniques to be used. In addition, the method provides DW and T2*W
images that are inherently co-registered with respect to both subject motion
and residual distortions caused by the EPI readout.

Acknowledgements

The Authors are
grateful to Dr Robert Frost and the Welcome Centre For Integrative Neuroimaging
at Oxford University for providing source code for their Slice-Grappa
implementation. Thanks are also due to Klaus Eickel for helpful discussions
during the course of this work.

Figures

Figure 1: Pulse diagram of the rs-EPI sequence for SMC
imaging. Data are acquired from two slice positions at the same time. One slice is generating DW contrast and the
other slice provides T2*-weighted contrast. A second echo is used to sample a
2D navigator region at the center of k-space
to correct the shot-to-shot, motion-induced phase variation for higher b-values
in DWI.

Figure 2: The upper rows show
the collapsed SMC data from DWI (b=0 and 1000 s/mm2) and
T2*-weighting with a FOV/2 shift in the T2*-weighted slices. In the lower rows
the corresponding separated slices are displayed respectively. The upper
section shows data without in-plane acceleration and the lower section SMC data
with an in-plane acceleration factor of two.

Figure 3: The trace-weighted
diffusion-weighted images from three different encoding directions (slice,
readout, phase-encoding) are shown from an SMC acquisition with no in-plane
acceleration (af = 1), an acceleration of two (af = 2), and four (af = 4). The
data without in-plane acceleration show some frontal lobe distortions and
signal losses in the ventricle region. These distortions are reduced when
in-plane acceleration is applied. However, with af = 4 SNR decreases visibly
and the current fat suppression works insufficient.

Figure 4: Here the averaged T2*W images calculated from the redundant
T2*W acquisition from all three diffusion-encoding directions are shown. Image
quality with an acceleration factor of two (af = 2) is sufficient in comparison
to no in-plane acceleration. However, the data acquired with an acceleration
factor of four (af = 4) show severe fat artifacts caused by an insufficient fat
suppression that needs to be adapted in the future.